1. Field of the Invention
The present invention relates to a method of evaluating tissue changes resulting from therapeutic hyperthermia by measuring the changes in the electrical impedance of the tissue.
2. Description of the Background
The electrical properties of cells and tissues have long been of interest to scientists. This has been particularly true since techniques for resistance and impedance measurements have become available. An early experiment by Hoeber demonstrated that very high frequencies and low frequencies yield different conductivity values for erythrocytes. This determination represented the first time that the existence of biological membranes was deduced from an electrical experiment.
The bulk electrical properties of tissues and cells are important as they determine the pathways of current flow through the body. Such properties are, therefore, important in the measurement of physiological parameters using impedance techniques, studies of biological effects of electromagnetic fields, electrocardiography, muscle contraction and nerve transmission.
Mammalian tissue may be viewed as a conglomeration of cells surrounded by membranes comprised of lipid and protein. These membranes resist the flow of ions and electric current thereacross and thus behave as electrical capacitive elements. Thus, intact healthy tissue resists electrical current. If the tissue is damaged such that the cells no longer maintain their membrane characteristics, the damaged membrane or damaged cells exhibit a reduced resistance or greater conductivity.
Recently, renewed interest in the use of hyperthermia for the treatment of cancer has considerably increased biological research on the molecular mechanisms and cellular alterations involved in heat-induced cell death. This relatively new cancer treatment technique, in which a volume of tissue in the body is heated to well above normal body temperature to promote the death of tumor cells, is presently being subjected to extensive laboratory investigation and clinical trials. Although hyperthermic techniques for the treatment of cancer have exhibited promise, the mechanism of heat-induced cellular alterations responsible for cell death remains unclear. Considerable evidence implies, however, that the cell membranes are one of the primary targets of hyperthermia damage.
Furthermore, in applying therapeutic hyperthermia in the clinic, an improper thermal dose can lead to severe clinical complications or to a lower probability of tumor control. At present, correlation of temperature measurements with laboratory findings or gross clinical data is the only method of assessing the thermal dose. Unfortunately, this means of correlation is frequently inadequate for an accurate prognosis.
Thus, a need continues to exist for a means by which the thermal dose to tissues in hyperthermic treatments can be accurately measured. Additionally, an accurate method is also needed for monitoring the progress of hyperthermic treatments.